Every wonder if you could make a water tight square keg? Check it out!

[JohnSand]

Not exactly. The lid has a beveled edge, that fits into a matching groove in the staves. The internal joints on the lids are flush, with thin shavings between the parts. They are held in place by edge nails, but made tight by the pressure from the hoops. If I were building one, all joints would be flush, the corners at 90', pressure applied by edge bolts. The bolts could be left a bit long for adjustment.

 

[GreenMonti]

Not exactly. The lid has a beveled edge, that fits into a matching groove in the staves. The internal joints on the lids are flush, with thin shavings between the parts. They are held in place by edge nails, but made tight by the pressure from the hoops. If I were building one, all joints would be flush, the corners at 90', pressure applied by edge bolts. The bolts could be left a bit long for adjustment.

I believe that's called a dado joint. If I remember right, a rabbit joint is on the edge of a board. I suppose I could google it.

How do you mean edge bolts?

 

[JohnSand]

I mean bolts that run through the pieces from side to side, through the width rather than the thickness. As if you laid the bolt flat across the plank, but inside. In boatbuilding, rudders, centerboards and centerboard trunks are built this way for strength and water-tightness. Even a modest bolt can apply great pressure. At the corner they would go through the adjacent side in the usual fashion, perhaps with an external reinforcing piece. The joints would have to be finished perfectly flush and smooth. I believe that the very thin slices between the staves are cedar or white pine. Compressed by pressure, and swelling quickly when wet, they help make the barrel tight. In traditional boats this would be cotton driven in between planks.
Sorry if some of my terms are not clear. I am not a pro, and boat carpentry, which I know best, uses different descriptions.

 

[GreenMonti]

Gotcha. Would you go all the way through with the bolts or would you stop part way?

I googled edge bolts in rudders to make sure my mental picture was what you were talking about. I was close. Based on the pics that popped up. From your description I envisioned a bolt going through the whole panel compressing all the planks together. Which would help with any sort of bowing that the panel might want to do once wet.

Edit:
I want to say an engineer at work told me a 1/4" bolt would carry 1,000 or 1,500 lbs. as long as the load was not in shear. Just linear to its length.

 

[JohnSand]

I would bolt them all the way through to seal the joint at the adjacent side too.